OsNPR1 negatively regulates herbivore‐induced JA and ethylene signaling and plant resistance to a chewing herbivore in rice

NPR1 (a non‐expressor of pathogenesis‐related genes1) has been reported to play an important role in plant defense by regulating signaling pathways. However, little to nothing is known about its function in herbivore‐induced defense in monocot plants. Here, using suppressive substrate hybridization, we identified a NPR1 gene from rice, OsNPR1, and found that its expression levels were upregulated in response to infestation by the rice striped stem borer (SSB) Chilo suppressalis and rice leaf folder (LF) Cnaphalocrocis medinalis, and to mechanical wounding and treatment with jasmonic acid (JA) and salicylic acid (SA). Moreover, mechanical wounding induced the expression of OsNPR1 quickly, whereas herbivore infestation induced the gene more slowly. The antisense expression of OsNPR1 (as‐npr1), which reduced the expression of the gene by 50%, increased elicited levels of JA and ethylene (ET) as well as of expression of a lipoxygenase gene OsHI‐LOX and an ACC synthase gene OsACS2. The enhanced JA and ET signaling in as‐npr1 plants increased the levels of herbivore‐induced trypsin proteinase inhibitors (TrypPIs) and volatiles, and reduced the performance of SSB. Our results suggest that OsNPR1 is an early responding gene in herbivore‐induced defense and that plants can use it to activate a specific and appropriate defense response against invaders by modulating signaling pathways.     

Mitogen-activated protein kinase OsMPK6 negatively regulates rice disease resistance to bacterial pathogens

Mitogen-activated protein kinase (MAPK) cascades play important roles in diverse developmental and physiological processes of plants, including pathogen-induced defense responses. Although at least 17 rice MAPKs have been identified and more than half of these MAPK genes have been shown to be pathogen or elicitor responsive, the exact role of most of the MAPKs in host-pathogen interaction is unknown. Here we report that OsMPK6 is an important regulator in rice disease resistance. Suppressing OsMPK6 or knocking out of OsMPK6 enhanced rice resistance to different races of Xanthomonas oryzae pv. oryzae, causing bacterial blight, one of the most devastating diseases of rice worldwide. The resistant plants showed increased expression of a subset of defense-responsive genes functioning in the NH1 (an Arabidopsis NPR1 orthologue)-involved defense signal transduction pathway. These results suggest that OsMPK6 functions as a repressor to regulate rice defense responses upon bacterial invasion. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reiterative and interruptive signaling in induced plant resistance to chewing insects

Our understanding of induced resistance against herbivores has grown immeasurably during the last several decades. Based upon the emerging literature, we argue that induced resistance represents a continuum of phenotypes that is determined by the plant’s ability to integrate multiple suites of signals of plant and herbivore origin. We present a model that illustrates the range of signals arising from early detection through herbivore feeding, and then through subsequent plant generations.     

SHP-2 positively regulates myogenesis by coupling to the Rho GTPase signaling pathway

Myogenesis is an intricate process that coordinately engages multiple intracellular signaling cascades. The Rho family GTPase RhoA is known to promote myogenesis, however, the mechanisms controlling its regulation in myoblasts have yet to be fully elucidated. We show here that the SH2-containing protein tyrosine phosphatase, SHP-2, functions as an early modulator of myogenesis by regulating RhoA. When MyoD was expressed in fibroblasts lacking functional SHP-2, muscle-specific gene activity was impaired and abolition of SHP-2 expression by RNA interference inhibited muscle differentiation. By using SHP-2 substrate-trapping mutants, we identified p190-B RhoGAP as a SHP-2 substrate. When dephosphorylated, p190-B RhoGAP has been shown to stimulate the activation of RhoA. During myogenesis, p190-B RhoGAP was tyrosyl dephosphorylated concomitant with the stimulation of SHP-2's phosphatase activity. Moreover, overexpression of a catalytically inactive mutant of SHP-2 inhibited p190-B RhoGAP tyrosyl dephosphorylation, RhoA activity, and myogenesis. These observations strongly suggest that SHP-2 dephosphorylates p190-B RhoGAP, leading to the activation of RhoA. Collectively, these data provide a mechanistic basis for RhoA activation in myoblasts and demonstrate that myogenesis is critically regulated by the actions of SHP-2 on the p190-B Rho GAP/RhoA pathway.     

A cold-induced thioredoxin h of rice, OsTrx23, negatively regulates kinase activities of OsMPK3 and OsMPK6 in vitro

Cytosolic thioredoxins are small conserved proteins that are involved in cellular redox regulation. Here, we report that a major and cold-induced thioredoxin h of rice, OsTrx23, has an inhibitory activity on stress-activated mitogen-activated protein kinases (MAPKs), OsMPK3 and OsMPK6 in vitro. This inhibition effects were redox-dependent and did not involve stable physical interaction. The data suggested a novel mechanism for redox regulation of MAPKs in plants.

Structured summary

MINT-7234362: MPK3 (uniprotkb:Q10N20) phosphorylates (MI:0217) MBP (uniprotkb:P02687) by protein kinase assay (MI:0424)MINT-7234435: MPK6 (uniprotkb:Q336X9) phosphorylates (MI:0217) MBP (uniprotkb:P02687) by protein kinase assay (MI:0424)     

Host plant defense signaling in response to a coevolved herbivore combats introduced herbivore attack

Defense‐free space resulting from coevolutionarily naïve host plants recently has been implicated as a factor facilitating invasion success of some insect species. Host plants, however, may not be entirely defenseless against novel herbivore threats. Volatile chemical‐mediated defense signaling, which allows plants to mount specific, rapid, and intense responses, may play a role in systems experiencing novel threats. Here we investigate defense responses of host plants to a native and exotic herbivore and show that (1) host plants defend more effectively against the coevolved herbivore, (2) plants can be induced to defend against a newly‐associated herbivore when in proximity to plants actively defending against the coevolved species, and (3) these defenses affect larval performance. These findings highlight the importance of coevolved herbivore‐specific defenses and suggest that naïveté or defense limitations can be overcome via defense signaling. Determining how these findings apply across various host–herbivore systems is critical to understand mechanisms of successful herbivore invasion.     

PTEN,a widely known negative regulator of insulin/PI3K signaling,positively regulates neuronal insulin resistance

Lipid and protein tyrosine phosphatase, phosphatase and tension homologue (PTEN), is a widely known negative regulator of insulin/phosphoinositide 3-kinase signaling. Down-regulation of PTEN is thus widely documented to ameliorate insulin resistance in peripheral tissues such as skeletal muscle and adipose. However, not much is known about its exact role in neuronal insulin signaling and insulin resistance. Moreover, alterations of PTEN in neuronal systems have led to discovery of several unexpected outcomes, including in the neurodegenerative disorder Alzheimer''s disease (AD), which is increasingly being recognized as a brain-specific form of diabetes. In addition, contrary to expectations, its neuron-specific deletion in mice resulted in development of diet-sensitive obesity. The present study shows that PTEN, paradoxically, positively regulates neuronal insulin signaling and glucose uptake. Its down-regulation exacerbates neuronal insulin resistance. The positive role of PTEN in neuronal insulin signaling is likely due to its protein phosphatase actions, which prevents the activation of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK), the kinases critically involved in neuronal energy impairment and neurodegeneration. Results suggest that PTEN acting through FAK, the direct protein substrate of PTEN, prevents ERK activation. Our findings provide an explanation for unexpected outcomes reported earlier with PTEN alterations in neuronal systems and also suggest a novel molecular pathway linking neuronal insulin resistance and AD, the two pathophysiological states demonstrated to be closely linked.     

Elevated CO2 reduces the resistance and tolerance of tomato plants to Helicoverpa armigera by suppressing the JA signaling pathway

Both resistance and tolerance, which are two strategies that plants use to limit biotic stress, are affected by the abiotic environment including atmospheric CO(2) levels. We tested the hypothesis that elevated CO(2) would reduce resistance (i.e., the ability to prevent damage) but enhance tolerance (i.e., the ability to regrow and compensate for damage after the damage has occurred) of tomato plants to the cotton bollworm, Helicoverpa armigera. The results showed that elevated CO(2) reduced resistance by decreasing the jasmonic acid (JA) level and activities of lipoxygenase, proteinase inhibitors, and polyphenol oxidase in wild-type (WT) plants infested with H. armigera. Consequently, the activities of total protease, trypsin-like enzymes, and weak and active alkaline trypsin-like enzymes increased in the midgut of H. armigera when fed on WT plants grown under elevated CO(2). Unexpectedly, the tolerance of the WT to H. armigera (in terms of photosynthetic rate, activity of sucrose phosphate synthases, flower number, and plant biomass and height) was also reduced by elevated CO(2). Under ambient CO(2), the expression of resistance and tolerance to H. armigera was much greater in wild type than in spr2 (a JA-deficient genotype) plants, but elevated CO(2) reduced these differences of the resistance and tolerance between WT and spr2 plants. The results suggest that the JA signaling pathway contributes to both plant resistance and tolerance to herbivorous insects and that by suppressing the JA signaling pathway, elevated CO(2) will simultaneously reduce the resistance and tolerance of tomato plants.     

OsCPK20 positively regulates Arabidopsis resistance against Pseudomonas syringae pv. tomato and rice resistance against Magnaporthe grisea

Calcium-dependent protein kinases are important decoders of calcium signals in plants, which are involved in plant immunity. We report isolation and functional characterization of a pathogen-responsive OsCPK20 gene in rice. The expression of OsCPK20 in rice was significantly induced following treatment with a Magnaporthe grisea elicitor. Overexpression of constitutively active OsCPK20 in Arabidopsis enhanced the resistance to infection with Pseudomonas syringae pv. tomato, associated with elevated expression of both SA- and JA-related defense genes. Similarly, transgenic rice plants containing constitutively active OsCPK20 exhibited enhanced resistance to blast fungus M. grisea. The enhanced resistance in the transgenic Arabidopsis and rice was associated with activated expression of both SA- and JA-related defense genes. We also found that OsCPK20 was significantly induced by drought stress, indicating that OsCPK20 might be involved in plant response to drought stress. Taken together, our results indicate that rice OsCPK20 positively regulates Arabidopsis resistance against Pseudomonas syringae pv. tomato and rice resistance against M. grisea, and that it may enhance disease resistance by activating both SA- and JA-dependent defense responses.     

Strigolactone signaling regulates rice leaf senescence in response to a phosphate deficiency

Strigolactones (SLs) act as plant hormones that inhibit shoot branching and stimulate secondary growth of the stem, primary root growth, and root hair elongation. In the moss Physcomitrella patens, SLs regulate branching of chloronemata and colony extension. In addition, SL-deficient and SL-insensitive mutants show delayed leaf senescence. To explore the effects of SLs on leaf senescence in rice (Oryza sativa L.), we treated leaf segments of rice dwarf mutants with a synthetic SL analogue, GR24, and evaluated their chlorophyll contents, ion leakage, and expression levels of senescence-associated genes. Exogenously applied GR24 restored normal leaf senescence in SL-deficient mutants, but not in SL-insensitive mutants. Most plants highly produce endogenous SLs in response to phosphate deficiency. Thus, we evaluated effects of GR24 under phosphate deficiency. Chlorophyll levels did not differ of in the wild-type between the sufficient and deficient phosphate conditions, but increased in the SL-deficient mutants under phosphate deficiency, leading in the strong promotion of leaf senescence by GR24 treatment. These results indicate that the mutants exhibited increased responsiveness to GR24 under phosphate deficiency. In addition, GR24 accelerated leaf senescence in the intact SL-deficient mutants under phosphate deficiency as well as dark-induced leaf senescence. The effects of GR24 were stronger in d10 compared to d17. Based on these results, we suggest that SLs regulate leaf senescence in response to phosphate deficiency.     

OsDMI3‐mediated activation of OsMPK1 regulates the activities of antioxidant enzymes in abscisic acid signalling in rice

In rice, the Ca²⁺/calmodulin (CaM)‐dependent protein kinase (CCaMK) OsDMI3 has been shown to be required for abscisic acid (ABA)‐induced antioxidant defence. However, it is not clear how OsDMI3 participates in this process in rice. In this study, the cross‐talk between OsDMI3 and the major ABA‐activated MAPK OsMPK1 in ABA‐induced antioxidant defence was investigated. ABA treatment induced the expression of OsDMI3 and OsMPK1 and the activities of OsDMI3 and OsMPK1 in rice leaves. In the mutant of OsDMI3, the ABA‐induced increases in the expression and the activity of OsMPK1 were substantially reduced. But in the mutant of OsMPK1, the ABA‐induced increases in the expression and the activity of OsDMI3 were not affected. Pretreatments with MAPKK inhibitors also did not affect the ABA‐induced activation of OsDMI3. Further, a transient expression analysis in combination with mutant analysis in rice protoplasts showed that OsMPK1 is required for OsDMI3‐induced increases in the activities of antioxidant enzymes and the production of H₂O₂. Our data indicate that there exists a cross‐talk between OsDMI3 and OsMPK1 in ABA signalling, in which OsDMI3 functions upstream of OsMPK1 to regulate the activities of antioxidant enzymes and the production of H₂O₂ in rice.     

Evidence for a disease-resistance pathway in rice similar to the NPR1-mediated signaling pathway in Arabidopsis

The Arabidopsis NPR1/NIM1 gene is a key regulator of systemic acquired resistance (SAR). Over-expression of NPR1 leads to enhanced resistance in Arabidopsis. To investigate the role of NPR1 in monocots, we over-expressed the Arabidopsis NPR1 in rice and challenged the transgenic plants with Xanthomonas oryzae pv. oryzae (Xoo), the rice bacterial blight pathogen. The transgenic plants displayed enhanced resistance to Xoo. RNA blot hybridization indicates that enhanced resistance requires expression of NPR1 mRNA above a threshold level in rice. To identify components mediating the resistance controlled by NPR1, we used NPR1 as bait in a yeast two-hybrid screen. We isolated four cDNA clones encoding rice NPR1 interactors (named rTGA2.1, rTGA2.2, rTGA2.3 and rLG2) belonging to the bZIP family. rTGA2.1, rTGA2.2 and rTGA2.3 share 75, 76 and 78% identity with Arabidopsis TGA2, respectively. In contrast, rLG2 shares highest identity (81%) to the maize liguleless (LG2) gene product, which is involved in establishing the leaf blade-sheath boundary. The interaction of NPR1 with the rice bZIP proteins in yeast was impaired by the npr1-1 and npr1-2 mutations, but not by the nim1-4 mutation. The NPR1-rTGA2.1 interaction was confirmed by an in vitro pull-down experiment. In gel mobility shift assays, rTGA2.1 binds to the rice RCH10 promoter and to a cis-element required sequence-specifically for salicylic acid responsiveness. This is the first demonstration that the Arabidopsis NPR1 gene can enhance disease resistance in a monocot plant. These results also suggest that monocot and dicot plants share a conserved signal transduction pathway controlling NPR1-mediated resistance.     

Brassinosteroid signaling positively regulates abscisic acid biosynthesis in response to chilling stress in tomato

Brassinosteroids (BRs) and abscisic acid (ABA) are essential regulators of plant growth and stress tolerance. Although the antagonistic interaction of BRs and ABA is proposed to ensure the balance between growth and defense in model plants, the crosstalk between BRs and ABA in response to chilling in tomato (Solanum lycopersicum), a warm-climate horticultural crop, is unclear. Here, we determined that overexpression of the BR biosynthesis gene DWARF (DWF) or the key BR signaling gene BRASSINAZOLE-RESISTANT1 (BZR1) increases ABA levels in response to chilling stress via positively regulating the expression of the ABA biosynthesis gene 9-CIS-EPOXYCAROTENOID DIOXYGENASE1 (NCED1). BR-induced chilling tolerance was mostly dependent on ABA biosynthesis. Chilling stress or high BR levels decreased the abundance of BRASSINOSTEROID-INSENSITIVE2 (BIN2), a negative regulator of BR signaling. Moreover, we observed that chilling stress increases BR levels and results in the accumulation of BZR1. BIN2 negatively regulated both the accumulation of BZR1 protein and chilling tolerance by suppressing ABA biosynthesis. Our results demonstrate that BR signaling positively regulates chilling tolerance via ABA biosynthesis in tomato. The study has implications in production of warm-climate crops in horticulture.     

The role of plant genotype,environment and gender in resistance to a specialist chrysomelid herbivore

Summary Patchiness in herbivore attack is a well-documented phenomenon. When neighboring plants suffer vastly different levels of attack, then one suspects genotypic differences among plants to be the underlying mechanism. In this study, I use common garden experiments in two natural, but divergent, habitats at the Cedar Creek Natural History Area in central Minnesota to determine the role of plant genotype, environment and gender in plant resistance to a specialist herbivore. Resistance was measured by larval survivorship and weight. Eight clones ofRhus glabra were selected and 12 equal-aged ramets were dug up and planted in two gardens (each garden received 6 ramets per clone). First instarBlepharida rhois (Coleoptera: Chrysomelidae) larvae of known parentage were transferred to ramets and censused every other day. At the end of the experiment, larvae were collected and weighed. Analysis of variance was used to determine the importance of plant genotype, environment and gender on larval mortality and weight. The experiment was repeated in its entirety one month later. Both plant genotype and environment significantly affected larval survivorship in the first run of the experiment. No interactions were significant. Results from the second run indicated marginally significant genotype and environment main effects, and a genotype by environment interaction in larval survivorship. There was a significant genotype by environment interaction in larval weight on the same run. In neither run did clone gender have significant affects on resistance.